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HELEN DEAN KING 



majority of litters reared in later generations were cast by 

 females that had grown rapidly and bred at an early age, but 

 selection of litters to continue the strain was not based on the 

 early breeding of the mothers, but on the size and vigor of 

 the young at birth, the number of individuals in a litter, and 

 its sex composition. 



In rats maintained under laboratory conditions reproduc- 

 tion ends, usually, when the animals are about 18 months of 

 age, although cases are known where young were born when 



Average age rl a 



Fig. 7 Average age of females when the first and the last litters were cast. 



females were over 2 years old. The time at which the meno- 

 pause appears depends largely upon the physical status of 

 the females, especially on their ability to resist pneumonia. 

 This infection soon ends reproductive activity, although af- 

 fected individuals may live for some months. 



The age at which individual females in later generations 

 stopped breeding varied greatly, the range being 185 days 

 (table 7). The average age of females at menopause tended 

 to advance with the generations, as is indicated by the course 

 of the upper graph in figure 7. 



